The invention relates to a process for producing an aluminum alloy strip by means of a strip casting machine, such that the said strip is suitable for can lid manufacture.
Can lids, in particular for beverage can bodies made of aluminum or steel, are mostly made of aluminum alloys. The most widely used process for manufacturing such beverage can lids is as follows.
The aluminum alloy AA 5182 containing the following main alloying constituents 4.4% magnesium, 0.3% manganese, 0.3% iron and 0.15 silicon is continuously chill cast as 30-40 cm thick ingots. These ingots are scalped, homogenized and hotrolled in several passes to a thickness of 2-3 mm. This strip is then usually annealed and cold rolled to an end thickness of 0.25-0.35 mm. Often the final rolled strip is subjected to a slight softening treatment at 170.degree.-200.degree. C. in order to prevent the strip from distorting during the paint baking. Before shaping into can lids the strip is coated with paint on both sides and then baked at 190.degree.-220.degree. C., typically 8 minutes at 204.degree. C.
As the recycling of aluminum is gaining in importance, in the USA more than half of all the used aluminum cans are returned for remelting, efforts have been made for some time now to develop an alloy which is equally suited for can bodies and can lids or at least can be made so after only small corrections to the common scrap from both lid and can body. In this connection the amount of primary aluminum required should in particular be as little as possible. This is not the case for the conventional alloys viz. AA 5182 for can lids and AA 3004 for can bodies as the alloy AA 3004 contains 1% magnesium, 1% manganese, 0.45% iron, 0.25% silicon and 0.15% copper, so that the resultant scrap contains approximately 1.6% magnesium, 0.7% manganese, 0.4-0.5% iron, 0.25% silicon, 0.1% copper and over 0.05% titanium.
Known from the U.S. Pat. No. 3,787,248 is a process which should make it possible to produce aluminum cans and lids from the same alloy. This alloy contains essentially 0.4-2.0% magnesium and 0.5-2.0% manganese. The process for manufacturing can lid material comprises continuous DC casting, homogenizing, hot rolling and subsequent cold rolling and annealing operations.
Known from the U.S. Pat. No. 4,235,646 is an economically attractive process for producing from one single aluminum alloy strip suitable for manufacturing deep drawn and ironed can bodies and can lids. This alloy contains essentially 1.3-2.5% magnesium and 0.4-1.0% manganese and can be made from the conventional can scrap without substantial addition of primary aluminum. The process for manufacturing the can lid stock comprises strip casting, hot rolling and cold rolling, the solidification rates employed being at the average level for example in the Hazelett or Alusuisse Caster II strip casters where the solidification takes place between casting belts or caterpillar track molds.
To save material, efforts are being made to reduce the thickness of the can lid. To meet the same requirements in terms of rigidity of the lid therefore both changes in design and a considerable increase in the strength of the material are necessary. With the above mentioned processes, however, these possibilities are limited.
In addition, the search for less expensive processes continues further.